Helical antenna elements encapsulated in plastic are well known. Some examples include U.S. Pat. No. 5,341,149 which describes an antenna rod with an antenna lead encapsulated in a layer of polymer material. U.S. Pat. No. 5,469,177 describes a helical antenna with a shaft translatable between a retractable position and a protractible position. PCT WO 95/08853 patent application describes a helical antenna with a variable reactance tuner. U.S. Pat. No. 4,725,395 describes a method for producing a helical antenna wherein a solid dielectric material is injection molded into a coil and an outer cover is injection molded over the coil and dielectric material.
The prior art suffers from several problems. It is desirable to completely encapsulate the helical antenna element in plastic, because the prior art has shown that this improves mechanical properties, particularly resistance to bend and impact. It is further desirable to maintain constant pitch and length of the helical coil constant during the encapsulation process, so as to produce an antenna with consistent frequency response. The prior art suggests several solutions to accomplish this. It has been found in practice, however, that maintaining the pitch and length within the required limits does not always result in antennas with the required frequency response. It is common knowledge that a thin helical antenna has narrow band width. Typically, tuning operations are applied to the helical coil before or after the encapsulation to fine tune the frequency response. However, fine tuning of the helical coil element is a cumbersome operation because the material of the element is typically a hard steel and it is difficult to cut a small amount of wire.